At least three people have been found to be carrying the Zika virus in east Yorkshire.
Calderdale and Huddersfield NHS Foundation Trust said the patients were believed to have contracted the virus overseas.
Zika has been declared a global public health emergency by the World Health Organisation (WHO).
The virus, which is mainly spread by the Aedes aegypt species of mosquito, typically causes only mild symptoms in adults but can cause a major birth defect called microcephaly in the babies of pregnant women who are infected.
Babies with microcephaly are typically born with abnormally small skulls – which can cause the child to suffer convulsions and develop learning difficulties.